The present invention relates to print band timing and more particularly to a detector circuit for precisely determining the time at which a timing mark on the print band passes a sensor responsive to the mark.
As is understood by those skilled in the printing art, many high speed computer printers utilize type fonts which are carried by a moving band. To effect printing, a hammer is actuated as a desired character on the band passes the column position where printing of that character is desired. In order to effect this sort of printing operation without skewing of the characters, it is necessary to precisely determine the position of the band. For this purpose, it is typical for the band itself to carry raised points or bars whose passage can be detected by a magnetic sensor and which therefore can act as timing marks. These raised points or bars are typically formed at the same time and by the same process which forms the font on the band.
Magnetic sensing is usually preferred for the band timing function since the print band typically operates in a dusty and dirty environment in which optical sensing would be difficult. Typically, sensing of the timing marks is accomplished magnetically by a magneto resistor though other types of magnetic sensors might also be used. In general, such sensors generate a bipolar pulse in response to the passing of one of the timing marks.
Since the length and magnitude of the bipolar pulse can vary depending upon many factors, e.g. the material of the particular band, the size of the mark, the spacing between the band and the sensor and the amount of wear experienced by the band, it has been found that most accurate operation is obtained if timing is taken from the mid-point transistion of the bipolar waveform. Heretofore, the magnetic sensors used for this purpose have typically been connected in a bridge circuit which was then balanced through resistor selection or adjustment in the final working magnetic environment. As will be understood by those skilled in the art, such individual adjustment or calibration is relatively time consuming and expensive in the context of an efficient manufacturing operation.
Among the several objects of the present invention may be noted the provision of a novel print band timing system; the provision of such a system in which the time of passing of a print band timing mark may be precisely determined; the provision of such a system which employs conventional timing marks and magnetic sensors which generate a bipolar pulse in response to the passing of a print band timing mark; the provision of such a system which precisely determines the nominal zero crossing of the bipolar pulse generated by the passing of a print band timing mark; the provision of such a system which is relatively insensitive to noise and d.c. drift; the provision of such a system which is highly reliable and which is of relatively simple and inexpensive manufacture. Other objects and features will be in part apparent and in part pointed out hereinafter.